1. Field of the Invention
This invention relates to methods and apparatus for correcting the aberration produced when a hologram is reconstructed using a light source displaced with respect to the hologram differently than the position of the reference beam during construction of the hologram and more particularly to a holographic method of producing a correction plate for use in the process to eliminate the aberrations.
2. Prior Art
Holograms are formed by photographically recording a coherent interference pattern between an object beam of light and a reference beam, and after development of the hologram light wavefronts which are the equivalent of the original object beam may be reconstructed by illuminating the hologram with a reconstructing light beam. The reconstructed light wavefronts will be distorted with respect to the object wavefronts if the nature of the reconstructing beam differs from that of the reference beam. In most situations this distortion is avoided by employing the same geometry for reconstruction as was used for formation of the hologram, but in certain circumstances the distortion which results from a modification of this geometry may be desirable.
For example, my U.S. Pat. No. 4,012,150 discloses a sight useful for purposes such as aiming guns, employing a hologram which records a single line of light. In use, the line reconstructed from the hologram appears to extend away from the observer toward a point located at infinity. Since it is virtually impossible to form the hologram by using a model line which extends in the same manner as the reconstructed line, the patent discloses a method of forming the hologram of a relatively short line, i.e., one or two meters long, which may be conveniently formed in a laboratory. During reconstruction the apparent position of the reconstructing beam is modified with respect to the apparent position of the reference beam during formation of the hologram to effectively enlarge the reconstructed light line relative to the original line used to form the hologram. This enlargement extends the far end of the line to a very large distance, virtually infinity, with respect to the observer.
This process of enlargement of the image reconstructed from a hologram relative to the image of an object used to form the hologram, by modifying the reconstructing geometry, results in a class of undesirable distortions or aberrations in addition to the desired image enlargement. These aberrations were analyzed and classified in a paper by Edwin B. Champagne entitled A Qualitative and Quantitative Study of Holographic Images, Technical Report AFALTR-67-107, Air Force Avionics Laboratory, Wright-Patterson Air Force Base, Ohio, July, 1967. This paper provides formulas for quantifying the spherical aberration, coma, and astigmatism produced by modifying the reconstructing geometry relative to the taking geometry for a hologram.
In terms of this light line sight, these aberrations result in parallax between the end of the light image and an object viewed through the hologram even if both are at the same distance as the observer, and a variation in the apparent distance from the hologram to the far end of the line image as the observer views the line image through different parts of the hologram. These effects reduce the accuracy of the light line if it is used as a sight or an instrument for range measurement.
The present invention is addressed to the solution of this problem by the provision of a method and apparatus for eliminating the aberrations at at least one point in an enlarged holographic image.